1. Field of the Invention
The present invention relates to anionic polymerization diinitiators and a process for preparing same. The present invention particularly relates to such initiators that are useful for the production of vinyl aromatic block copolymers.
2. Background of the Art
The use of anionic initiators to prepare block copolymers is well known. In a synthetic method, an initiator compound is used to start the polymerization of one monomer. The reaction is allowed to proceed until all of the monomer is consumed resulting in a living homopolymer. To this living homopolymer is added a second monomer that is chemically different from the first. The living end of the first polymer serves as the site for continued polymerization, thereby incorporating the second monomer as a distinct block into the linear polymer. The block polymer so grown is living until terminated.
Termination converts the living end of the block copolymer into a non-propagating species, thereby rendering the polymer unreactive toward additional monomers. A polymer so terminated is commonly referred to as a diblock copolymer. Initiators are commonly monofunctional. That is they have only one site that can initiate polymerization so that, in effect, the initiator is at one end of the polymer chain that builds in a single direction, that direction being away from the initiator. Exemplary monoinitiators include, for example, sec-butyl lithium.
Preparing a polymer using a diinitiator can offer advantages over preparing a similar polymer using a monoinitiator. By using a diinitiator, the polymer can be grown in both directions at the same time, thereby reducing the polymerization time. But the use of such diinitiators can also be troublesome. For example, U.S. Pat. No. 3,734,973, to Farrar, describes the production of anionic polymerization diinitiators by reacting diisopropenylbenzene compounds with organo monolithium compounds and then carrying out anionic copolymerization of styrene and butadiene using the diinitiators.
Unfortunately, the chemistry involved in the reaction of the diisopropenylbenzene and the lithium alkyl is prone to suffer from a competing side reaction that forms oligomers of diisopropenylbenzene. The oligomers consume diisopropenylbenzene and this limits the yield of the dilithium initiator. In addition and perhaps more importantly, the oligomers have more than two lithium centers per molecule. If all of the lithium centers in the oligomers initiate polymerization, a nonlinear, star, or radial polymer will result. The branching in such star and radial polymers leads to an increase in the melt viscosity of the polymer. This is undesirable if the desired polymer is a linear, difunctional anionic polymer.
One possible solution to the problem of oligomers having multiple initiation sites is disclosed in U.S. Pat. No. 6,217,798 B1 to Willis, et al. Therein it is disclosed that combining the components in a particular order, carrying out the reaction within a narrow temperature range, and carrying out the reaction in the presence of the appropriate amount of diethyl ether provides the advantage of minimizing the oligomerization of the diisopropenyl-benzene.
While the diinitiators prepared according to U.S. Pat. No. 6,217,798 B1 are clearly superior to the prior art diinitiators, they are not trouble free and require both time and resources to produce. It would be desirable in the art of preparing diinitiators for use in anionic polymerization to develop diinitiators that are more stable, less nucleophilic and less basic, and to prepare diinitiators that have primary anionic polymerizations sites of substantially equal reactivity.